Method for the preparation of n-propyl bromide

ABSTRACT

A method is described for producing n-propyl bromide of a high degree of purity, which contains isopropyl bromide in an amount lower than 0.1% w/w, and usually lower than 0.05%. The method is characterized in that n-propanol is reacted with HBr which is in gas form, preferably dry, and which is in excess over the stoichiometric amount, wherein the excess HBr is at the end of the reaction in an aqueous solution formed from the reaction water. The invention further relates to N-propyl bromide of high purity, containing typically less than 500 ppm of isopropyl bromide.

FIELD OF THE INVENTION

This invention relates to a method for the production of n-propylbromide which provides said compound with a high degree of purity and inparticular, with an extremely low content of isopropyl bromide.

BACKGROUND OF THE INVENTION

N-propyl bromide is a product which has considerable industrialimportance, particularly as a solvent for cleaning and degreasingreplacing ODS (such as: 1,1,1-TCE, CFC-113) used in precision cleaningelectronic defluxing, metal cleaning, adhesives, coatings, products andsolvent applications.

In principle, n-propyl bromide is the product of a reaction betweenhydrobromic acid HBr and n-propanol. However, depending on the way inwhich the reaction is carried out, the result may be unsatisfactory,e.g., as to conversion and purity of the product, and most particularly,because of the contamination with by-products, especially isopropylbromide.

Schematically, the reaction is as follows:HBr+CH₃—CH₂—CH₂OH→CH₃—CH₂—CH₂Br+H₂O.

In prior art processes, the n-propyl bromide form contains a weightpercentage of isopropyl bromide which may approach and even exceed 1%.(All the percentages in this specification and claims are by weight,unless otherwise indicated.) Said percentage is higher than desirable,because isopropyl bromide has a proven toxic effect, and it is a purposeof this invention to provide a process for the preparation of n-propylbromide from HBr and n-propanol wherein the resulting n-propyl bromidecontains amounts of isopropyl bromide equal or below 0.1%, preferablyequal or below 0.05%, viz. equal or below 500 ppm.

It is another purpose of this invention to provide such a process whichcan be carried out continuously or in batch procedure.

It is a still further purpose of this invention to provide such aprocess which has a high conversion rate.

Other purposes and advantages of this invention will appear as thedescription proceeds.

SUMMARY OF THE INVENTION

The process for the production of n-propyl bromide according to theinvention comprises reacting hydrogen bromide with n-propanol, andcomprises feeding HBr which is in gas form and preferably dry. Waterforms in the reaction and if an excess of HBr over the stoichiometric isfed, the excess HBr will be in an aqueous solution in the reactionwater. According to a preferred feature of the invention, said aqueoussolution should have, at the end of the reaction, a concentration of HBrnot lower and preferably higher than 48%, and more preferably aconcentration of 50-52%, or as high as 62%, viz. near saturation at roomtemperature. The molar excess of HBr required to achieve the desiredfinal concentration of the HBr in aqueous solution can easily bedetermined by an easy stoichiomeric calculation. Generally, said excessmay vary, e.g., from 20 mol % to 50 mol %, but these are not limitativevalues. The term “dry” in relation to HBr means in this application acontent of water not higher than 1% w/w. The process of the invention,however, can be carried out with HBr that is in gas form but containswater in excess of 1% w/w.

The n-propanol may be liquid or gaseous. In one embodiment of thereaction—which is generally a batch process, but can be carried outcontinuously by feeding the reagents and withdrawing the productcontinuously—gaseous HBr is fed into liquid n-propanol, which is kept atthe reaction temperature, which is not less than 40° C. and preferably,from 45 to 150° C. An aqueous phase and an organic phase are formed inthe process and separate, the organic phase consisting of n-propylbromide and the aqueous phase consisting of an HBr solution, at a weightconcentration of HBr e.g. from 48 to 62%.

It should be understood that, in some embodiments of the process,portions of the aqueous phase may be separated as soon as formed, out ofcontact with the organic phase, and if so, the HBr concentration of suchportions is not critical and may be lower than 48%.

In another embodiment of the invention, which is a continuous process,gaseous HBr and n-propanol vapors are fed to a reaction space—generallya column—in which a catalyst is present, and which is kept at atemperature above the boiling point of n-propanol, preferably above 100°C. The reaction product is discharged from the reaction space to a coldcondenser. The condensed product again comprises an organic phase and anaqueous phase, which is an aqueous solution of HBr. The two phases arecollected and allowed to separate. However, the separation occurs onlyafter the end of the reaction, and therefore during the reaction theorganic phase and the aqueous phase are always in contact and theconcentration of the HBr solution must be within the limits hereinspecified.

The amount of isopropyl bromide in the reaction product, when thereaction is carried out according to the invention, is below 0.1%,usually considerably below 0.05 wt %, e.g., between 100 and 450 ppm. Ann-propyl bromide having such a low amount of isopropyl bromide is notknown in the art and is not achievable by the processes known in theart, and therefore, constitutes in itself an aspect of the invention.

As has been said, the process of the invention can be carried out inbatch or continouosly.

The batch process is preferably performed in a stirred reactor equippedwith a reflux condenser, a thermometer and a dip-pipe used for theaddition of the HBr. Heating and cooling are supplied by a bath. It isalso possible to circulate the contents of the reactor through a packedcolumn in countercurrent to the HBr stream.

The continuous process is preferably performed in packed columnscontaining various brands of active carbon. Heating to the desiredtemperature is supplied to the upper two thirds of the column by anexternal heating tape. Two feed pipes, one for n-propanol and the otherfor HBr, are connected to the top of the column. The reagents are co-fedat calculated weight ratios, reacting while flowing downwards throughthe hot area to the lower cold area where they condense, to be collectedin a cold vessel. Temperatures at various points along the column aremeasured with thermocouple probes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following examples are illustrative and not limitative.

EXAMPLE 1 Batch Reaction Between 100% HBr Gas and N-Propanol

N-propanol (120 Kg) is fed into a reactor and heated to 45° C. GaseousHBr is fed at constant temperature during about 21 hours (at a rate of11.4 Kg/hr and about 239 Kg). The molar ratio HBr/n-propanol is about1.5:1. At the end of the HBr feeding period, the mixture is heated to60-65° C. for an additional 2 hours, then cooled to ambient temperature.The two phases formed during the process are separated. N-propyl bromide(d=1.35) is the upper phase. The lower phase is a ˜50% HBr solution inthe reaction water, said solution having a density of 1.55. The twophases are separately discharged from the reactor. The upper, n-propylbromide phase (170 liters), is evaporated and dried. Its content ofisopropyl bromide, before evaporation, is 100-200 ppm. The lower 50% HBrsolution phase has a volume of 90 liters. Though the solubility ofn-propyl-bromide in such an aqueous solution is low, a small amount ofn-propyl bromide is recovered by distillation from said solution.

The characteristics of the n-propyl bromide product are: GC: 99.8-99.9%;Water: 100-300 ppm; Color: 5 Apha; Isopropyl bromide: 200-300 ppm.Yield, based on n-propanol: 95%. The concentration of isopropyl bromidein the product is slightly higher than its concentration in the organicphase withdrawn from the reactor, because during the evaporation of theorganic phase there are removed some impurities, such as dipropylether,some unreacted propanol and some heavy by-products.

EXAMPLE 2 Continuous Reaction Between 100% HBr Gas and N-Propanol Vapor

Into the top of a column of 25 mm diameter and 33 cm length, packed withactive carbon (F-300 by Chemviron), n-propanol vapors (above 100° C.)and 100% HBr gas are fed simultaneously at an 1:1.37 molar ratiorespectively. The column is kept at an average temperature of 102-120°C. The rate of alcohol addition is 50 gr/hour. After two hours, thecrude product (202 gr) contained less than 500 ppm of isopropyl bromide.Here an aqueous phase of 55% HBr is collected, along with the organicproduct. The n-propyl bromide is washed with water and neutralized. Itshould be noted that in the reaction in which propanol is in the gasphase, an excess of HBr significantly lower than that used when thepropanol is liquid suffices to give the desired low isopropyl bromidepercentage in the product. It is believed that this is due to the factthat the gases are cooled to ambient temperature as soon as they emergefrom the reaction zone and no more isomerization to isopropyl bromidetakes place.

Characteristics of the n-propyl bromide product: GC: 99.7%; Water: ND;Color: ND; Isopropyl bromide: 450 ppm; Yield, based on n-propanol:98.1%.

EXAMPLE 3 Batch Reaction Between 100% HBr Gas and N-Propanol UnderPressure

n-Propanol (600 g) was fed into a 1.3 liter pressure reactor. Into thesealed reactor, HBr gas (100%, 1100 g) was fed at 80° C. The totaladdition time was 80 minutes. The pressure in the reactor rose to amaximum of 2.5 atm. The reactor was cooled to ambient temperature andthe lower aqueous phase (464 g of 62% HBr) was separated. The organicphase was neutralized by washing with 20% sodium hydroxide, yieldingn-propyl bromide (1210 g). Analysis: 99.7% n-propyl bromide, isopropylbromide below 0.05%.

In the following comparative examples, aqueous solutions of HBr are usedfor the reaction with n-propanol. Examples 4, 5 and 6 are comparativeexamples. Examples 4 and 5 illustrate liquid phase reactions, andExample 6 a gas phase reaction not according to the invention, since theHBr is fed as an aqueous solution and not as a gas.

EXAMPLE 4 Batch Reaction Between 62% HBr Solution and N-Propanol

Both liquids are fed into a reactor at ambient temperature (8034 Kg and1269 Kg, respectively, molar ratio HBr/n-propanol 2.9:1.0). The stirredmixture is heated to 61-66° C. for 3 hours. After cooling, two phasesare separated at 30° C. The lower phase consists of 48% HBr, in which apart of the product is dissolved. The upper phase consists of n-propylbromide. The aqueous phase is heated up to 125° C. to recover thedissolved n-propyl bromide by evaporation. This fraction is combinedwith the main product. The product is washed with alkali and dried overanhydrous sodium carbonate.

Characteristics of the n-propyl bromide product: GC: 99% min.; Water:250 ppm; Color: 60 Apha; Isopropyl bromide: 0.14%. Yield, based onn-propanol: 96-97%.

EXAMPLE 5 Continuous Reaction Between 48% HBr Solution and N-Propanol

A mixture of 48% HBr and n-propanol is heated in a stirred reactor to122-123° C. More of both reagents is continuously fed via pumps inequimolar quantities, while removing simultaneously by distillation allthe n-propyl bromide and water formed in the process. The concentrationof the HBr is kept constant at 48% during the whole process. Thus, at anaverage rate of 4.5 Kg n-propanol/hour feeding (with parallel feeding ofHBr), a crop of 345 Kg of n-propyl bromide was collected within ca. 37hours of operation.

Characteristics of the n-propyl bromide product: GC: 98.2-98.7%; Water:ND; Color: ND; Isopropyl bromide: 0.6-1.1%; Yield, based on n-propanol:95%.

EXAMPLE 6 Continuous Reaction Between 48% HBr and N-Propanol in GasPhase

Into the top of a column packed with active carbon (¾″ diameter and 40cm long), n-propanol vapors (at 100° C.) and vaporized 48% HBr (122° C.)are fed simultaneously at an equimolar ratio. The HBr feed is keptconstant at 278 gr/hour of 48% solution. The product collected at thelower part of the column consists (after cooling) of two phases: theupper one is mainly the depleted HBr containing some unreactedn-propanol, and the lower, n-propyl bromide. The conversion based onn-propanol is about 90%, and that based on HBr is about 82%.

Characteristics of the n-propyl bromide product: GC: 97-99%; Water:100-300 ppm; Color: ND; Isopropyl bromide: 0.8-1.6 wt %; Yield, based onn-propanol; 90%.

It is seen that when an aqueous solution of HBr is used, the productcontains at least 0.14 wt % of isopropyl bromide and in most cases,more, e.g., up to 1.6 wt %. This occurs whether the reaction is carriedout in batch form as a continuous reaction, and in this latter casewhether the HBr solution is fed as such or is vaporized, and withdifferent concentrations of aqueous HBr solution.

The following table illustrates the comparison between the process ofthe invention and the prior art. No. Reagents Phase n-PrBr % i-PrBr % 1100% HBr gas and n-propanol liquid 99.8 0.03 2 100% HBr gas andn-propanol gas 99.7 0.045 3  62% HBr and n-propanol liquid 99 0.5 4  48%HBr and n-propanol liquid 98-98.7 0.6-1.1 5  48% HBr and n-propanol gas97-99 0.8-1.6

The catalyst used in the continuous gaseous reaction according to theinvention may be different from the active carbon mentioned in Example2. Other carbons or different catalysts can be used, e.g. RD3 or RB1produced by Norit Nederland B.V. of 3800 Ac Amersfoort, The Netherlands,RB1 by Norit, granular activated carbon by Darco, F-300-Chemviron Carbonof Zoning Industriel C. de Feluy, B-7181 Feluy, Belgium. The temperaturerange can also vary. A wide range of temperatures may be used: 97-160°C. at atmospheric or superatmospheric pressure.

While embodiments of the invention have been described by way ofillustration, it will be understood that the invention may be carriedinto practice with many modifications, variations and adaptations,without departing from its spirit or exceeding the scope of the claims.

1. Process for the production of n-propyl bromide by reacting hydrogenbromide with n-propanol, which is characterized in that the HBr is ingas form and is in such an excess over the stoichiometric amount as toproduce at the end of the reaction an aqueous solution of HBr in thereaction water having a concentration of at least 48%.
 2. Processaccording to claim 1, wherein the HBr is dry.
 3. Process according toclaim 1, wherein the HBr is in such an excess over the stoichiometricamount that the aqueous solution of HBr in the reaction water has aconcentration of at least 50%.
 4. Process according to claim 1, whereingaseous HBr is in molar excess over the stoichiometric of at least 20%with respect to n-propanol.
 5. Process according to claim 1, carried outas batch process.
 6. Process according to claim 5, carried out byfeeding gaseous HBr into liquid n-propanol.
 7. Process according toclaim 1, carried out as continuous process.
 8. Process according toclaim 7, carried out by feeding gaseous HBr and n-propanol vapors to areaction space, in which a catalyst is present and which is kept at atemperature above the boiling point of n-propanol.
 9. Process accordingto claim 1, wherein an inorganic phase and an organic phase are formed,the organic phase consisting of n-propyl bromide and the inorganic phaseconsisting of an HBr solution, and which process comprises cooling thereaction product and separating the two phases and recovering then-propyl bromide.
 10. Process according to claim 1, carried out in areaction space the temperature of the reaction space being above 100° C.11. Process according to claim 8, wherein the catalyst is chosen fromthe group consisting of active carbon and granular activated carbon. 12.N-propyl bromide, characterized in that it comprises an amount ofisopropyl bromide below 0.1%
 13. N-propyl bromide, characterized in thatit comprises an amount of isopropyl bromide below 0.05%.